1. Field of the Invention
The present invention relates to a wire electrode for wire cutting electro-discharge machining, and more particularly, it relates to a wire electrode for wire cutting electro-discharge machining which comprises a core and a coating layer provided on the outer periphery of the core.
2. Description of the Background Art
Wire cutting electro-discharge machining is a method of generating an electric spark discharge across a workpiece and a wire-type working electrode (hereinafter referred to as "wire electrode") and relatively moving the wire electrode and the workpiece for cutting the workpiece into a desired shape. Such electro-discharge machining has been carried out in general.
In general, a long wire electrode 0.05 to 0.25 mm in diameter is passed through a guide so that a new wire electrode is successively supplied to a worked portion or electro-discharge location, thereby carrying out electro-discharge machining.
In such electro-discharge machining, the quality of the wire electrode that is employed directly, remarkably influences the processing speed, processing accuracy, surface smoothness of a worked surface and the like. Thus, the development of a preferable material suitable for such electro-discharge machining is greatly desired in the field.
An example of a conventional wire electrode is made of a material prepared by adding Al, Si and the like to a Cu-Zn alloy. Zn, Al, Si and the like are added in order to improve the processing speed. If the total amount of Zn (wt. %)+(6.times.Al)(wt. %)+(9.times.Si)(wt. %) exceeds 38 percent by weight, however, it is difficult to work the material into a wire electrode.
When a current applied to the wire electrode is increased, the processing speed is improved. If the total amount of Zn (wt. %)+(6.times.Al)(wt. %)+(9.times.Si)(wt. %) exceeds 38 percent by weight, however, the heat resistance of the material is so deteriorated that the wire electrode disadvantageously becomes fused when a large current is applied there-to. Thus, the current cannot be increased and hence it is impossible to improve the processing speed.
Japanese Patent Publication No. 57-5648 (1982) and Japanese Patent Laying-Open Nos. 59-129629 (1984) and 59127921 (1984) disclose other examples of conventional wire electrodes. Each of the wire electrodes disclosed in these references has a two-layer structure of a core and a coating layer which is formed on the outer periphery of the core.
The core is made of a copper alloy, while the coating layer is made of a zinc alloy. The coating layer is mainly composed of zinc since the processing speed is increased in proportion to the content of zinc. The zinc contained in the coating layer is evaporated when electricity is applied to the wire electrode, to allow smooth discharge across the wire electrode and a workpiece. Such smooth discharge leads to improvement of the processing speed.
When the coating layer is mainly composed of a zinc alloy in order to improve the processing speed, however, this coating layer is made fragile. When the wire electrode is passed through a guide, therefore, the coating layer may be scraped by friction between the guide and the coating layer.